Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode separated by an electrolyte. The anode receives hydrogen gas and the cathode receives oxygen, typically via an air flow. The hydrogen gas is dissociated at the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. In a PEMFC, the electrolyte may be a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. In one preferred form, the bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates may also include flow channels through which a cooling fluid flows.
Various techniques are known in the art for fabricating the bipolar plates. In one design, the bipolar plates are made of a composite material, such as graphite, where two plate halves are separately molded and then glued together so that anode flow channels are provided at one side of one of the plate halves, cathode flow channels are provided at an opposite side of the other plate half and cooling fluid flow channels are provided between the plate halves. In another design, two separate plate halves are stamped and then welded together so that anode flow channels are provided at one side of one of the plate halves, cathode flow channels are provided at an opposite side of the other plate half and cooling fluid flow channels are provided between the plate halves.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, the water may accumulate within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the relatively hydrophobic nature of the plate material. The droplets form in the flow channels substantially perpendicular to the flow of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because a typical stack may be configured to have at least some of the fuel cells electrically coupled in series, if one of the fuel cells stops performing, the operation of the entire fuel cell stack may be jeopardized.
A fuel cell stack typically includes a seal that extends around the active area of the fuel cells between the stack headers and the active area for each fuel cell to prevent gas leakage from the stack. Therefore, in order to get the cathode flow, the anode flow and the cooling fluid flow from the respective inlet header into the active area of the fuel cell, it is necessary for the flow channels to go through the seal area without affecting seal integrity. Typically holes or tunnels are provided through the bipolar plate around the seals, which requires a bend in the flow channels so that they line up with the flow channels in the active area. This bend in the cathode and anode flow channels provided an area that water could accumulate and be trapped which has a tendency to close the flow channel and reduce the flow of reactant gas thereto. Therefore, a better technique for traversing the seal area of the fuel cell stack is needed.
A common configuration for the Unitized Electrode Assembly, UEA, of a fuel cell has an overlap region between a thrifted membrane (a membrane sized to fit short of the plate perimeter seal) and sub-gasket (a “window frame” of polymer film such as polyethylene naphthalate, PEN) at the perimeter of the active area. Membrane thrifting is done for a few reasons: 1) the membrane material is not compatible to be in contact with the seal or plate material due to its high acidity, so the membrane needs to be covered or separated by a plastic film in the fuel cell seal region; 2) the membrane material is expensive, so by thrifting the membrane material from the perimeter of the cell, less membrane material is required; 3) the membrane material is not compatible with water-glycol coolant due to high membrane swelling, so the membrane needs to be pulled in from the coolant header. However, this configuration creates an interface between the membrane and sub-gasket at the perimeter of the active area that must be sealed. In particular, it is important that the active area perimeter along the flow-directional edge be sealed, whereas it is important that coolant, anode and cathode flow along the perimeter of the active region orthogonal to the flow-directional edge not be impeded as they traverse the overlap region. Due to the composition and dimensional change with hydration of the membrane, adhesive bonding between the membrane and sub-gasket is not reliable. To ensure reliable sealing in this overlap region, mechanical pressure is necessary.
In this regard, stamped plate design poses greater challenges than molded plate design as the stamped features are necessarily reflected on both sides of the stamped plate half. For molded plates, regions can be filled with solid composite material as needed. For example, the land of a stamped plate always creates a coolant channel behind it, while a molded plate can be solid to avoid creating coolant channels where they are not desired. Another consideration is that providing good mechanical support in the overlap region is more critical for fuel cells using thinner diffusion media (DM), as thinner DM is less able to distribute compression load across channel spans. Thinner DM is desirable to reduce the size of the fuel cell which is particularly important for packaging into automotive applications. Hence, an improved seal design is needed for sealing the perimeter of an active area of a fuel cell manufactured with stamped plates.